How do you go from 70% of NBS to 15%?

The Herald reports:

Onslow College said the gym previously had a seismic rating of 70% of the NBS and O Block had a rating of 55%. …

Gym (Block Q): An overall NBS rating of 15%, with the ceiling braces posing the greatest risk.

I’m not an engineer but as a lay person I don’t understand how a building can be assessed at 70% of NBS one moment and then 15% the next.

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